Nowadays, lightweight, non-shattering, transparent resin materials are widely used as a substitute for transparent flat glass. For example, plastic substrates, especially polycarbonate resins, due to their excellent properties including transparency, impact resistance, and heat resistance, are currently used as structural members in place of glass, in various applications including windows in vehicles and buildings, meter covers and the like. However, it is strongly desired to improve the surface properties of plastic substrates, typically polycarbonate resins because their surface properties like mar resistance and weatherability are poor as compared with glass. Known means for improving the mar resistance of molded polycarbonate resins is by coating the surface of a resin substrate with a thermosetting resin such as organopolysiloxane. Known means for improving weatherability is by providing the surface of a resin substrate with a weather-resistant resin layer containing a UV absorber.
In the prior art, substrates of organic resins or plastics are surface coated with various coating compositions to form surface protective films for the purpose of imparting high hardness and mar resistance. For instance, compositions comprising hydrolyzates or partial hydrolyzates of hydrolyzable organosilanes and optionally, colloidal silica are known. For example, JP-A S63-168470 discloses a coating composition comprising a hydrolyzate and/or partial condensate of an organoalkoxysilane and colloidal silica, wherein the alkoxy group is converted into silanol in the presence of excess water.
Although coatings resulting from such coating compositions have a high hardness and provide sufficient protection for plastic substrates, they lack toughness. Specifically, a thick film of at least 10 μm is prone to crack upon heat curing or upon abrupt temperature changes during outdoor service. Also these coatings have a poor UV screening ability so that the interface between a primer layer (for imparting adhesion to the plastic substrate) and the topcoat layer may be degraded by UV. As a result, the coating may be stripped off and the plastic substrate may be discolored.
JP-A H08-151415 describes that a coating composition comprising a mixture of a benzotriazole or benzophenone base UV-absorptive vinyl monomer and a vinyl monomer copolymerizable therewith is applied onto the surface of a synthetic resin substrate to form a protective coating. It is also known from JP 3102696 that a resin article with a multilayer coating which imparts weather resistance while remaining adhered to the resin substrate is obtainable using a coating composition comprising a copolymer of a benzotriazole or benzophenone base UV-absorptive vinyl monomer, an alkoxysilyl-containing vinyl monomer, and a vinyl monomer copolymerizable therewith. These protective coatings, however, lack mar resistance since they are based on vinyl polymers. It is also a practice in the art to add an organic UV absorber to a silicone layer. However, merely adding the UV absorber to a coating composition leads to low sustainability. That is, the durability of the composition in coating form is poor in that the UV absorber will bleed out and run on the surface after long-term exposure.
It is then proposed to use a silyl-modified organic UV absorber capable of forming a chemical bond with a siloxane compound as the base of a coating. See JP-B H03-14862, JP-B H03-62177, and JP-A H07-278525. This proposal improves sustainability since the UV absorber is strongly bound to the siloxane matrix. On the other hand, these coatings are significantly degraded in the essentially desired mar resistance under the influence of UV absorptive functional group incorporated for weatherability, or develop noticeable microcracks due to a lowering of flexibility.
The method of manufacturing a transparent article having both weatherability and mar resistance is described in JP-A S56-92059 and JP-B H07-10966. Known are UV absorbing transparent articles in which a protective coating of a colloidal silica-containing polysiloxane composition is formed on a transparent substrate via a primer layer having a high loading of UV absorber. JP 3102696 discloses a coated article which is provided with mar resistance and weather resistance by forming a primer layer of a copolymer composition, and forming a colloidal silica-containing polysiloxane resin coating thereon. However, since the topcoat layer does not at all possess weather resistance, the interface between the primer layer and the topcoat layer can be degraded by UV, tending to raise problems like separation between the primer layer and the topcoat layer. The coated article is thus insufficient in long-term weatherability.
As discussed above, a number of attempts have been made to improve the weather resistance, mar resistance and other properties of plastic substrates coated with coating compositions. However, there is not available an article coated with a coating composition which exhibits mar resistance and UV screening property and meets sufficient weather resistance and durability to withstand prolonged outdoor exposure while maintaining transparency to visible light.